This application is based on patent application Nos. H10-104359 (104359/1998) Pat. and H11-56061 (56061/1999) Pat. both filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a display device including a liquid crystal display element, and particularly a display device including a liquid crystal display element having a liquid crystal display layer which includes liquid crystal and resin. The invention also relates to a method of driving a liquid crystal display element, and particularly a method of driving a liquid crystal display element having a liquid crystal display layer which includes liquid crystal and resin.
2. Description of the Background Art
A liquid crystal display element including liquid crystal material which exhibits a cholesteric characteristic does not require a polarizer, and can perform a bright reflective display because it utilizes selective reflection of incident light by the liquid crystal material. Further, it can perform a high resolution display by simple matrix driving without requiring a memory element such as TFT or MIM.
When driving the liquid crystal display element which includes the liquid crystal material exhibiting the cholesteric characteristic by application of the voltage, two kinds of, i.e., high and low pulse voltages are applied to the liquid crystal layer for switching the orientation state of the liquid crystal molecules between the planar orientation state and the focal conic orientation state. When the liquid crystal layer is supplied with a high pulse voltage, which can orient the helical axes of the liquid crystal along the electric field direction and thus can achieve a homeotropic orientation, the liquid crystal will enter the planar state, in which the helical axes of the liquid crystal molecules forming each domain are perpendicular to the substrate, after the high pulse voltage application stops. When the liquid crystal layer is supplied with a low pulse voltage, which cannot achieve the complete homeotropic state of the liquid crystal, the liquid crystal material will enter the focal conic state, in which the helical axes of the liquid crystal molecules forming each domain are oriented irregularly or substantially parallel with the substrate, after the low pulse voltage application stops. The planar state and the focal conic state are stably held even after stop of the voltage application.
The liquid crystal material exhibiting the cholesteric characteristic selectively reflects the rays of a wavelength corresponding to a product of the helical pitch and the average refractive index of the liquid crystal material when it is in the planar orientation. Therefore, by employing the liquid crystal materials of which selective reflection wavelengths are in a red range, a blue range and a green range, respectively, the liquid crystal materials in the planar orientation selectively reflect the rays of the respective wavelengths to perform display in red, blue and green. When the liquid crystal material exhibiting the cholesteric characteristic has a short helical pitch, for example, has such a short helical pitch that the selective reflection wavelength is in a visible range or below the visible range, the liquid crystal material in the focal conic state scatters the visible rays to a less extent so that a nearly transparent appearance can be exhibited.
Accordingly, by employing the liquid crystal material, which has a selective reflective wavelength in the visible range and exhibits the cholesteric characteristic, together with a black background, and by switching the state between the planar state and the focal conic state, the display in the selective reflection state (planar state) and the black state (focal conic state) can be selectively performed.
By employing the liquid crystal material having the selective reflection wavelength, e.g., in an infrared range, the liquid crystal material in the planar state exhibits a transparent appearance because it reflects only the infrared rays, i.e., the rays of the selective reflection wavelength, and allows passage of visible rays and others. In this case, the helical pitch is relatively long so that the liquid crystal material in the focal conic state scatters the incident rays to exhibit an opaque appearance.
Accordingly, by using the liquid crystal material, which has the selective reflection wavelength in the infrared range and exhibits the cholesteric characteristic, together with the black background, the display in black (planar state) and white (focal conic state) can be selectively performed by switching the state between the planar state and the focal conic state.
In the liquid crystal display element including twist nematic liquid crystal material, supertwist nematic liquid crystal material or the like, the state of liquid crystal material changes in accordance with the effective value of the drive voltage. Therefore, the simple matrix driving cannot achieve a practically sufficient contrast if the pixels are large in number. However, the liquid crystal display element including the liquid crystal material which exhibits the cholesteric characteristic has the memory property as already described, and therefore, can be driven by the simple matrix driving to achieve a practically sufficient contrast even if the pixels are large in number.
U.S. Pat. No. 5,384,067 has disclosed the following prior art. A liquid crystal display element having a liquid crystal composite film, which is formed of polymerized and phase-separated chiral nematic liquid crystal and resin, is supplied with a pulse voltage for driving. The pulse voltage has a magnitude intermediate the voltage, which can set the whole liquid crystal material in the composite film to the planar state, and the voltage, which can set the whole liquid crystal material to the focal conic state. The magnitude of this voltage is controlled so that the composite film attains the state, in which the domains in the planar state and the domains in the focal conic state are present in a mixed fashion, and thereby the gray-scale display can be performed.
In addition to the above, the following art has been studied. A liquid crystal display element having a composite film, which is formed of liquid crystal material exhibiting the cholesteric characteristic and resin, is supplied with a first pulse voltage having a magnitude achieving the homeotropic state, in which the molecules of the liquid crystal are oriented parallel with the electric field. After a predetermined time from the application of the first pulse voltage, a second pulse voltage is applied for stabilizing the composite film. The magnitude of the second pulse voltage is controlled so that display in intended levels can be performed.
However, when employing the method of driving the liquid crystal display element, in which multiple-tone display is performed by controlling the magnitude of the pulse voltage, as disclosed in U.S. Pat. No. 5,384,067 as well as the method of driving the liquid crystal display element by applying the first and second pulse voltages, expensive analog ICs are required in drive circuits connected to the liquid crystal display elements, and therefore the display device is expensive as a whole.
An object of the invention is to provide a liquid crystal display device using a liquid crystal display element, and particularly to provide a liquid crystal display device which can inexpensively perform display in multiple tone levels.
Another object of the invention is to provide a liquid crystal display device using liquid crystal display element, which is provided with liquid crystal display layer including liquid crystal material exhibiting a cholesteric characteristic, and particularly to provide a liquid crystal display device which can inexpensively perform display in multiple tone levels.
Still another object of the invention is to provide a method of driving a liquid crystal display element which can inexpensively perform display in multiple tone levels.
Yet another object of the invention is to provide a method of driving a liquid crystal display element, which is provided with a liquid crystal display layer including liquid crystal material exhibiting a cholesteric characteristic, and particularly to provide a method of driving a liquid crystal display element which can inexpensively perform display in multiple tone levels.
The invention provides a liquid crystal display device comprising a liquid crystal display element having a first substrate provided with a plurality of scanning electrodes, a second substrate provided with a plurality of signal electrodes and a liquid crystal display layer held between the first and second substrates; and a drive voltage applying device for applying a scanning voltage to the scanning electrodes and applying a signal voltage to the signal electrodes, wherein
the drive voltage applying device applies a first pulse voltage to the scanning electrode corresponding to a drive target pixel in the liquid crystal display layer for changing the liquid crystal material of the target pixel to a predetermined changed state;
applies, subsequently to the first pulse voltage, a second pulse voltage to the scanning electrode corresponding to the target pixel as well as a third pulse voltage to the signal electrode corresponding to the target pixel in synchronization with the second pulse voltage for stabilizing the state of the liquid crystal material of the target pixel in a predetermined stabilized state;
and controls a pulse width of the third pulse voltage in accordance with required display tone of the target pixel.
The invention also provides a liquid crystal display device comprising a liquid crystal display element having a first substrate provided with a plurality of scanning electrodes, a second substrate provided with a plurality of signal electrodes and a liquid crystal display layer held between the first and second substrates; and a drive voltage applying device for applying a scanning voltage to the scanning electrodes and applying a signal voltage to the signal electrodes, wherein
the drive voltage applying device applies a first pulse voltage to the scanning electrode corresponding to a drive target pixel in the liquid crystal display layer for changing the liquid crystal material of the target pixel to a predetermined changed state;
applies, subsequently to the first pulse voltage, a second pulse voltage to the scanning electrode corresponding to the target pixel as well as a third pulse voltage having a pulse width equal to or larger than the pulse width of the second pulse voltage to the signal electrode corresponding to the target pixel in synchronization with the second pulse voltage for stabilizing the state of the liquid crystal material of the target pixel in a predetermined stabilized state;
and controls on-timing of the third pulse voltage with respect to on-timing of the second pulse voltage and/or off-timing of the third pulse voltage with respect to off-timing of the second pulse voltage to change the phase of the third pulse voltage with respect to the phase of the second pulse voltage within a range from a state where the second and third pulse voltages do not overlap with each other, to a state where the second pulse voltage is included in the third pulse voltage in accordance with required display tone of the drive target pixel.
The invention also provides a method of driving a liquid crystal display element having a first substrate provided with a scanning electrode, a second substrate provided with a signal electrode and a liquid crystal display layer held between the first and second substrates, the method comprising the steps of:
(a) applying a first pulse voltage to the scanning electrode corresponding to a drive target pixel in the liquid crystal display layer for changing the liquid crystal material of the target pixel to a predetermined changed state;
(b) applying, subsequent to the step (a), a second pulse voltage to the scanning electrode corresponding to the target pixel; and
(c) controlling a pulse width of a third pulse voltage in accordance with a required display tone of the target pixel, and applying the third pulse voltage to the signal electrode corresponding to the target pixel in synchronization with the second pulse voltage for stabilizing the state of the liquid crystal material of the target pixel in a predetermined stabilized state.
The invention further provides a method of driving a liquid crystal display element having a first substrate provided with a scanning electrode, a second substrate provided with a signal electrode and a liquid crystal display layer held between the first and second substrates, the method comprising the steps of:
(a) applying a first pulse voltage to the scanning electrode corresponding to a drive target pixel in the liquid crystal display layer for changing the liquid crystal material of the target pixel to a predetermined changed state;
(b) applying, subsequent to the step (a), a second pulse voltage to the scanning electrode corresponding to the target pixel; and
(c) applying a third pulse voltage having a pulse width equal to or larger than a pulse width of the second pulse voltage to the signal electrode corresponding to the target pixel, with controlling an on-timing of the third pulse voltage with respect to an on-timing of the second pulse voltage and/or an off-timing of the third pulse voltage with respect to an off-timing of the second pulse voltage to change a phase of the third pulse voltage with respect to a phase of the second pulse voltage within a range from a state where the second and third pulse voltages do not overlap with each other, to a state where the second pulse voltage is included in the third pulse voltage in accordance with a required display tone of the drive target pixel.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.